An estimated 40,000 people traveled from around the world to take up arms for the Islamic State group as it occupied territory in Syria and Iraq and declared a caliphate in 2014.

A few hundred are believed to still be fighting as IS struggles to survive, having lost most of its territory to campaigns by Western-backed Syrian and Iraqi coalition armies.

But what happened to the rest?

Many thousands were certainly killed in the intense fighting, but US experts believe many have survived, posing a formidable threat going ahead.

“The issue is: how many have died? How many are still there and willing to fight? How many have gone elsewhere to fight?” said Seth Jones, director of the International Security and Defense Policy Center at the Rand Corporation.

“How many have given up? I don’t think we have a good answer.”

International counterterror groups are putting huge efforts into answering those questions, working hard to name, count and track IS foreign fighters.

In France, officials say, around 1,700 people went to Iraq and Syria since 2013 to join IS. Of those, 400 to 450 have been killed, and 250 returned to France.

Foreign Affairs Minister Jean-Yves Le Drian said on December 8 that about 500 are still in the Iraq-Syria theater, and for them it is now very hard to return to France.

But that leaves another 500 whose whereabouts are unknown, many of them with the skills of war, wielding weapons and making bombs.

As the experience of the 1980s-2000s showed, the “Afghan” Arabs were a gift that kept on giving for the global jihad, despite of the fact that only a small minority of them came (and returned to) Western countries. The “Syrian” Arabs are likely to be more dangerous and more immediately so. We’re in for a rough ride ahead.